The present invention relates to an apparatus for positioning a moored, floating vessel around a turret.
In offshore production of oil and other subaqueous minerals, an alternate to bottom mounted towers that extend above the surface of the water is to provide a portion of the production facilities on the sea floor and to provide a permanently moored floating facility for the balance. When this is done, a substantially vertically extending conduit or riser must be provided between the sea floor and the floating facility. Mooring lines are used to anchor the facility to the sea floor. These mooring lines and risers which are connected to the vessel via a turret must be kept from being twisted when the vessel turns (i.e., windvanes) in response to wind, waves, and current forces.
U.S. Pat. No. 3,602,175 to Morgan discloses a vessel having a rotatable plug or mooring swivel therethrough near the bow for mooring to the bottom and accommodating a riser passing therethrough. The Morgan patent discloses a ball bearing system for the plug or swivel to keep the mooring lines and risers in the proper position as the rotatable plug swivels within the vessel. This pivoting means is integrally built into the mooring vessel.
U.S. Pat. No. 4,305,341 to Stafford discloses a spindle in a vessel wherein radial-thrust bearings at the top and bottom side portions of the spindle and vertical-thrust bearings along the side of the spindle permit the vessel to windvane about the spindle.
A problem with the bearing systems in the Morgan and Stafford patents is the flexing of the bearing surfaces. Depending on the loads placed on a vessel, the vessel can be in a xe2x80x9chogxe2x80x9d condition or a xe2x80x9csagxe2x80x9d condition. When a vessel is in a xe2x80x9chogxe2x80x9d condition, the deck of the vessel is in tension and has a convex surface. When a vessel is in a xe2x80x9csagxe2x80x9d condition, the deck of the vessel is in compression and has a concave surface. The xe2x80x9chogxe2x80x9d condition occurs when the vessel is not loaded and the xe2x80x9csagxe2x80x9d condition occurs when the vessel is fully loaded. However, the degree of xe2x80x9chogxe2x80x9d and xe2x80x9csagxe2x80x9d dynamically changes, based on the forces exerted on the vessel by the wind, waves, and ocean currents.
The result of the hog or sag condition is that the normally circular bearing housing will assume various elliptical forms as the vessel changes from compression to tension conditions alternately at the main deck and bottom. There are no known steel roller bearing arrangements that can accommodate this elliptical distortion condition and yet retain tolerable friction losses.
Clearly, there is a need for a turret bearing apparatus that will accommodate the hog and sag conditions and permit rotation of the vessel around a turret, such that the mooring lines are not twisted. Also, needed is an apparatus that dynamically adjusts its radial support of the turret.
An apparatus for rotating a floating vessel such as a ship, around a cylindrical turret structure moored to the sea bottom. The vessel has a circumturret structure passing through the hull of the vessel and surrounding the turret structure. The apparatus comprises a hook roller assembly, a load roller assembly, a radial roller assembly, and a drive mechanism. The hook roller assembly prevents the turret structure from moving upward from the wave and current forces exerted on the floating vessel. The load roller assembly rotatably supports the turret structure on the circumturret structure. The plurality of radial roller assemblies mounted to the circumturret radially support the turret. The drive mechanism is used to rotate the vessel around the turret.
The hook and load roller assemblies facilitate the rotation of the vessel around the turret. The hook and load roller assemblies comprise a pair of rails and a plurality of wheels interposed between the rails. For each assembly, one rail is attached to the turret structure and one rail is attached to the circumturret structure such that the rails are generally aligned with each other. The rails have a convex shaped rail head. Each wheel has an outer bearing surface with a concave curvature. The outer bearing surface of the wheel is in contact with each aligned pair of rails. However, because of the curvature of the outer bearing surface and the flanges on the wheels, when the rails become misaligned, there is enough curvature in the wheels to maintain sufficient surface contact between the outer bearing surface and the rail head of each rail, thereby facilitating the rotation of the vessel around the turret regardless of the alignment of the rails.
To support the turret in the radial direction, a radial roller support assembly is mounted to the circumturret structure. The radial support assembly has a circular rail mounted on the turret, a wheel, a wheel support assembly, a cushion pad, and a hydraulic cylinder assembly. The radial support assembly, (1) provides radial support to the turret regardless of the shape of the circumturret, and (2) equalizes the loads on the turret so that the cylindrical shape of the turret is preserved.
The drive mechanism in conjunction with the drive gear, cam, and cam follower rotate the vessel around the turret. The drive gear is connected to the turret structure. A motor having a pinion is pivotably connected to the circumturret structure. The pinion engages a drive gear tooth on the drive gear. A cam is integral to the drive gear with a cam follower connected to the circumturret vessel structure to counteract a separation force between the drive gear and the pinion. When the motor drives the pinion, the circumturret structure rotates in relation to the turret structure.